The present invention relates to implantable medical devices, and more particularly to the output stage of an implantable electrical stimulator. Even more particularly, the invention relates to a programmable output current source for use within an implantable tissue or nerve stimulator, e.g., an implantable cochlear stimulator.
In the implantable medical device field, a medical device, configured to perform a desired medical function, is implanted in the living tissue of a patient so that a desired function may be carried out as needed for the benefit of the patient. Numerous examples of implantable medical devices are known in the art, ranging, from implantable pacemakers, cochlear stimulators, spinal cord stimulators, muscle stimulators, glucose sensors, and the like.
Some implantable medical devices are configured to perform the stimulating function, i.e., to generate an electrical signal, typically a biphasic current pulse, that is applied to body tissue or to a nerve, for the purpose of causing a desired muscle contraction, or activating a desired nerve. Examples of medical devices that perform the stimulating function are shown, e.g., in U.S. Pat. Nos. 5,193,539 and 5,603,726, both of which patents are incorporated herein by reference. The present invention is directed to such implantable stimulating devices.
Other medical devices are configured to perform the sensing function, i.e., to sense a particular parameter, e.g., the amount of a specified substance in the blood or tissue of the patient, and to generate an electrical signal indicative of the quantity or concentration level of the substance sensed. An example of an implantable medical device that performs the sensing function is shown, e.g., in U.S. Pat. No. 4,671,288. Sometimes, sensing a desired parameter or other element, e.g., an evoked response, occurs through the same or similar electrodes through which stimulation is applied.
Some stimulating devices, e.g., cardiac pacemakers, and the microstimulators of the type disclosed in the above-referenced '539 patent, may always apply their stimulation pulses through the same electrodes. Other stimulating devices, e.g., cochlea stimulators of the type disclosed in the above-referenced '726 patent, however, apply the stimulus pulses, which are of varying amplitude and polarity, to different electrodes, or different electrode pairs, as a function of the sensed input signal. There is thus a need in the implantable stimulating device field to provide an output circuit for use with such devices that allows different electrode pairs to be selected through which stimulus currents having different amplitudes and polarities may be applied.
As medical devices have become more useful and numerous in recent years, there is a continual need to provide very low power sensors and stimulators that may be connected to, or incorporated within, such devices so that the desired function of the device can be carried out without the expenditure of large amounts of power (which power, for an implanted device, is usually limited). Moreover, there is a constant need and desire to make such implantable devices smaller and smaller.
To meet this need (smaller circuits, less power) within an implantable stimulating device, it has been common to design an output circuit for interfacing with the electrodes which selectively connects a single current source, or one of a plurality of current sources, to a selected electrode pair, rather than designing a separate current source for each electrode pair. In order to allow the polarity of the stimulus applied through the selected electrode pair to be changed without requiring an additional current source, which would tend to make the device larger and consume more power, it is also known to employ a switching matrix which selectively connects the selected electrode pair to either side of the current source, or which otherwise causes the current flow direction through the selected electrode pair to change. All such switching matrices, or equivalent schemes, require that the electrode nodes (those nodes connected directly to a given electrode) be physically or electrically switched from one circuit location to another. Disadvantageously, such "switching" requires additional circuitry which not only increases the impedance of the circuit connection path, but which also, during operation, takes time to complete, i.e., takes a finite time to switch the switching components from being biased ON (low impedance) or OFF (high impedance) so that the desired "connection" may be established or disconnected.
There is thus a need in the art for an output circuit useable within an implantable current stimulator that is small, operates at low power, and does not physically or electrically switch the electrode node from one circuit location to another as different and continually changing stimulus parameters (amplitude, polarity, electrode pairings) are needed.